An insulating oil for use in transformers and other electrical equipment has two important functions. First, it acts as an electrical insulating medium and second, it transports heat generated in the windings and core of the transformer or in connected circuits to cooling surfaces. In addition to possessing dielectric strength and cooling capacity, the ideal insulating oil should be environmentally safe and relatively nonflammable.
Hydrocarbon or mineral oils derived from crude petroleum oil were used extensively for many years as insulating oils in electrical equipment. However, such oils have been replaced to a great extent by less flammable oils. Since the 1930's the polychlorinated biphenyls (PCB) which are generally considered to be nonflammable have been widely utilized as replacements for mineral oils insulating oils in electrical equipment. Nonflammability is a required property for insulating oils to be used in equipment which is placed within and around building structures where it is necessary to minimize the hazard of fire and explosion damage in the event of electrical faults within the equipment.
In recent years, it has become generally recognized that polychlorinated byphenyls (PCB) are environmentally hazardous liquids. As a result, strict requirements have been established concerning the construction and the installation of equipment intended for use with such oils to prevent any fluid leakage in event of a catastrophic failure. In addition, the disposal of liquids or fluids containing PCB must now be made in compliance with the very exacting procedures outlined in the new environmental protection laws. Furthermore, the polychlorinated biphenyls because they lack the ability to extinguish internal arcing cannot be used in safety and operational devices such as submerged high voltage fuses, breakers and switches.
Because of the disadvantages and shortcomings of the polychlorinated biphenyls, there have been numerous efforts made to develop relatively inexpensive, environmentally safe, nonflammable insulating oils. To date these efforts have not been completely successful.
Representative of the prior art insulating oils are those disclosed in U.S. Pat. Nos. 3,000,807; 3,095,366; 3,406,111; 3,587,168 and 3,753,188.